This side-by-side comparison of four representative oil barrels illustrates the differences in the total amount of greenhouse-gas (GHG) emissions per barrel.1 But it also illustrates that different processes in oil’s supply chain contribute to each oil’s emissions in their own way. From this image, for example, we can see not only that a well-managed, conventional, lighter oil is overall much less emissions intensive than the other oils, but also that a relatively small proportion of its emissions come from extraction (upstream) and refining (midstream) processes. Of the four representative barrels shown, the one with the highest emissions upstream is the mismanaged light, gassy oil, while the depleted oil has the greatest midstream emissions and the extra-heavy oil has the highest downstream emissions. The next visualizations give us an opportunity to learn more about these four sample oils and the various processes that affect each of their emissions profiles.
The first barrel offers a proportioned emissions breakdown of the supply chain for a well-managed, conventional light oil that produces relatively little GHG emissions upstream and midstream; most of its emissions arise from the combustion of products like gasoline and bunker fuel. As such, greater vehicle efficiency and clean fuel substitution are generally the most effective ways to reduce the negative climate impact of this type of oil.
Oils from depleted fields—so called because they are often several decades or over a century old and can become saturated with water—are heavy and difficult to coax from the ground. This is due to the fact that over the life of an oil field, the easiest-to-extract oil is extracted first, well pressure diminishes, and chemical processes can increase the oil’s carbon-to-hydrogen ratio.
The most significant contributor of emissions for depleted oil is the production of steam necessary to make the viscous crude flow from underground. Moreover, when the depleted oil’s ratio of carbon to hydrogen is relatively high, refining it requires the addition of hydrogen—an energy-intensive process—as well as the rejection of excess carbon; this latter process creates a byproduct called petroleum coke (or petcoke). Petcoke is comparable to—and can be even worse than—coal in its climate and air-quality impacts when combusted downstream. To reduce emissions from heavier, depleted oils, strategies to consider include generating steam and hydrogen with renewables and developing non-combustible uses for petcoke.
The light, gassy oil pictured above is quite different from the other types of oil discussed above. As a light oil, its ratio of carbon to hydrogen is lower than most other oils, therefore requiring little to no hydrogen addition and no carbon rejection to make high-value petroleum products; petcoke is not produced and does not contribute to its downstream emissions at all. However, it is also a gassy oil, meaning that natural gas is either absorbed in the oil or sitting above it in the reservoir. When the oil is extracted, the gas is extracted as well; if there is no immediate use or equipment to collect and sell the gas, it can either be allowed to escape into the atmosphere (vented) or burned off at the well-head (flared). Another means of gas release involves fugitive emissions, whereby methane escapes—often accidentally and undetected during day-to-day operations—in smaller amounts from all oil equipment.
Methane, the most prevalent hydrocarbon of natural gas, is a short-lived but highly potent GHG, so venting is a particularly climate-damaging practice, and allowing fugitive emissions to go unchecked can also amount to significant damage over time; flaring generates large amounts of CO2 from the combustion of the gas. The more carefully the natural gas contained in light, gassy oils is managed so as to minimize venting, flaring, and fugitive emissions, the less damage these oils will do to the climate.
The most emissions intensive barrel overall, the extra-heavy oil, is physically distinctive and requires a transformation so that it flows more like a conventional crude and can be handled as such. Different pathways for such conversion are already available, while others are still under development. For example, upgrading extra-heavy oil to a synthetic crude requires an energy-intensive upstream process that rejects carbon and produces petcoke. The synthetic crude is then delivered to an appropriate refinery and processed like conventional crude. Another process involves diluting extra-heavy oil with lighter hydrocarbons to make the oil flow to a refinery, where a significant quantity of hydrogen is added and/or carbon is rejected. New extra-heavy oil pathways that reduce emissions from or otherwise avoid upstream upgrading, midstream refining hydrogen processing, and downstream combustion of petcoke are needed to reduce the climate impacts of this oil type.
In a paper to be published later this spring, we will elaborate further on some of the opportunities that private and public stakeholders have to reduce the climate challenges presented by the proliferation of new types of oil. Also, visit Carnegie’s expanding OCI web tool for the forthcoming release of OCI Phase Two, which will allow users to compare GHG emission estimates of 75 oils, representing 25 percent of global production, giving stakeholders the opportunity to learn a lot more about—and from—our changing oil resources.
1 The Oil-Climate Index (OCI) estimates the total emissions associated with a barrel of a sample oil by accounting for offsite emissions credits and debits. Credits are generated upstream if, for example, gas co-produced with oil is captured and utilized, thereby displacing the need to acquire other, more emissions intensive sources of power or exporting excess energy generated offsite. If upstream processes necessitate the import of electricity from offsite, then an emission debit is generated. The upstream emissions credit is particularly large for the light, gassy oil because the natural gas coproduced with the oil can be utilized in numerous ways, including to generate electricity at the well site or to export offsite and sold. Taking emissions credits and debits into account means that the most emissions intensive of these four sample types is the extra-heavy oil.